1988 CBR250 turning over but won't start, can't figure it out.

[FJRider]

Back in the day it was called an apprenticeship.

Now everyone wants their qualifications from google.

Back in the day ... "fixing" the bike yourself was the option of choice ... regardless of the issue. The Public Library had most of the workshop manuals (and photo-copiers) so some knowledge was available cheaply. And those rip-off workshop mechanic's charged over $10 an hour to look at them.

With the OP having access to THESE forums ... Not having internet is not the issue. They're just obviously too dam fucking lazy to even Google it.

[OddDuck]

Thanks for the suggestions OddDuck!

• I've emptied the tank and put fresh fuel in.
• I've checked all the sparks now and I'm sure they're ok.
• I can't see any holes or gaps around the air intake/filter.

Before I see if I can do your other suggestions we've found something...

Since posting two days ago a friend of mine came to have a look. He discovered that one of the throttle cables has snapped and is missing so I'm thinking this is mostly likely the problem.

From what I can see the top throttle cable pulls while the bottom cable pushes. The bottom cable is the one that's missing. My question now is are both cables absolutely necessary or is the top cable capable of doing the job on it's own?

Also when I try to start the bike or open the throttle the headers don't open. Could that be what the missing cable is for? Are they supposed to open when you turn the throttle or are they opened by a vacuum?

Broken throttle cable: the push pull arrangement is so that the rider can close the throttle in an emergency situation. One cable pulls when opening, the other pulls when closing. Neither cable pushes, they just don't work that way. Older bikes used a pull cable to open the throttle and relied on springs at the other end to close it again - all fine until the cable rusts or gunks up and jams in the open position. I'd take the view that it's there for a reason and I'd replace it, but up to you.

What you call the headers - are you talking about the carburettor slides?

There are two types of carburettor used on motorcycles:

CV - Constant Velocity - these move the slides automatically depending on engine demand - i.e they're opened by airflow. The throttle cables go to a butterfly valve mounting in the carburettor body, immediately downstream of the slides. You're almost certain to have CV's if the bike's stock.

Slide Carbs - no throttle butterfly, instead the slide is directly controlled via cable from the twistgrip.

Both types ideally also have a choke valve, this being either a butterfly valve upstream of the carburettor or an enricher which works on the float valve.

Anyway... last suggestion really, once you've got the cable sorted, try putting a voltmeter onto the battery and see how much the voltage at the terminals drops while cranking the starter motor. It should start at 12V and drop to 8 to 10V; if it's lower than this while cranking then that could be your problem.

If it's an old battery it'll drop storage capacity, even fully charged, and if there's not enough oomph left to run the ignition properly then no amount of cranking will get the motor to catch.

[stuartb]

With the OP having access to THESE forums ... Not having internet is not the issue. They're just obviously too dam fucking lazy to even Google it.

I've been trying to figure out the problem for a few months now. Which has included A LOT of Googling and reading old forum posts. Like I said in the first post, I'm new to bikes so I'm trying my best here and I'm keen to learn. I started this post asking for help as a last resort before re-assembling it and taking it to a mechanic.

You've got to be a bit of a dip shit to come on here just to assume and post I'm "too dam fucking lazy to even Google it".



Anyway...

Broken throttle cable: the push pull arrangement is so that the rider can close the throttle in an emergency situation. One cable pulls when opening, the other pulls when closing. Neither cable pushes, they just don't work that way. Older bikes used a pull cable to open the throttle and relied on springs at the other end to close it again - all fine until the cable rusts or gunks up and jams in the open position. I'd take the view that it's there for a reason and I'd replace it, but up to you.

What you call the headers - are you talking about the carburettor slides?

There are two types of carburettor used on motorcycles:

CV - Constant Velocity - these move the slides automatically depending on engine demand - i.e they're opened by airflow. The throttle cables go to a butterfly valve mounting in the carburettor body, immediately downstream of the slides. You're almost certain to have CV's if the bike's stock.

Slide Carbs - no throttle butterfly, instead the slide is directly controlled via cable from the twistgrip.

Both types ideally also have a choke valve, this being either a butterfly valve upstream of the carburettor or an enricher which works on the float valve.

Anyway... last suggestion really, once you've got the cable sorted, try putting a voltmeter onto the battery and see how much the voltage at the terminals drops while cranking the starter motor. It should start at 12V and drop to 8 to 10V; if it's lower than this while cranking then that could be your problem.

If it's an old battery it'll drop storage capacity, even fully charged, and if there's not enough oomph left to run the ignition properly then no amount of cranking will get the motor to catch.

Thanks for the info OddDuck. I've ordered a new throttle cable so I will replace that. Carburettor slides does sound like the proper name for what I'm talking about.

You must be right about them being CVs, I can see the butterfly values under the carburettor slides.

I'll give the battery another charge and see how much the voltage drops now.

[4AGE]

Fuel pump. Common fault on these.

[stuartb]

Fuel pump. Common fault on these.

Fuel pump is brand new and I've checked it's getting fuel through the pipes.

[husaberg]

does it have a sidestand switch

[WALRUS]

need to know a bit to keep those old jap 250s going now that theyve done a million kms at redline for most their life `_-

You're not wrong. I've got a pair of 1989 MC19 racebikes and yeah, I'm sure they were thrashed harder by teenagers learning on them as road bikes than they ever were on track.

Both of mine are currently in the same state as OP's where they are complete, fuel pump working fuel getting into carbs, got spark and turning over but not firing. I think my problem is just old and gummed up carbs but I don't have the time or know-how to service/clean/strip/re-build them.

Do you have good compression?

When was the last time StuartB cleaned his carbs?

Don't forget, as others have said, this is a 30yo small capacity bike which has done, I assume, many many thousands of kms at eleventy billion RPM with (often) rather poor maintenance and the odd stack or two.

One of my engines has a tendency of blowing headgaskets and doing weird shit

[FROSTY:Me]

probably way late but the most common issue with those lil bikes was fouled plugs.

[jafagsx250]

probably way late but the most common issue with those lil bikes was fouled plugs.

fouled plugs can make it drop cylinders and die right? Cos mine is giving me a ball ache with that shit.

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[jellywrestler]

My current mechanic of choice is a spade is a spade type of person.

The phrase predates the use of the word "spade" as an ethnic slur against African Americans,[9] which was not recorded until 1928; however, in contemporary U.S. society, the idiom is often avoided due to potential confusion with the slur


well he's a bit politically incorrect then isn't he?

[OddDuck]

fouled plugs can make it drop cylinders and die right? Cos mine is giving me a ball ache with that shit.

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Weak spark can do that - check to see how much voltage your coils are getting, if the wiring loom's getting old then the lengths of wire / multiple crimp joints / push connectors etc resistances can start to add up.

[Hugo Nougo]

not on here often, but a month and no one knows if its spark, fuel or compression. we can confirm however that spades have no place in the workshop.

[jellywrestler]

not on here often, but a month and no one knows if its spark, fuel or compression. we can confirm however that spades have no place in the workshop.

on the contrary it might have spade terminals.

[jellywrestler]

perhaps.

Didn't know it was a slur or anything like that, just goes to show how things change over time.

Like the word: Gay

The question is: did you understand what I wrote in context or are you going to twist it around to some other connection

yip exactly, just jumping on the pc bandwagen

[jafagsx250]

Weak spark can do that - check to see how much voltage your coils are getting, if the wiring loom's getting old then the lengths of wire / multiple crimp joints / push connectors etc resistances can start to add up.

They checked that. It was ok.

Turned out it was a swollen/perished fuel diaphragm on the fuel cock that was causing the fuel flow to be irregular.

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